Acrylic pressure-sensitive adhesive tapes are excellent in the adhesive force, the cohesive force, etc., and are used as fixing tapes, masking tapes, etc., for electronic parts. The pressure-sensitive adhesive tapes of this kind are usually produced by a method of solution-polymerizing a (meth)acrylic acid alkyl ester and, if necessary, other monomer such as acrylic acid in an organic solvent, compounding the polymer obtained with a tackifier resin and a crosslinking agent and the like, to prepare a pressure-sensitive adhesive, and after coating the pressure-sensitive adhesive on a base material or a releasing liner, removing the organic solvent by heating.
The acrylic pressure-sensitive adhesive sheets produced as described above are liable to cause discoloring such as yellowing under a high-temperature atmosphere or an exposure in the open air by, perhaps, the influence of the compounded component such as a crosslinking agent or in the pressure-sensitive adhesive sheets. Furthermore, there is a possibility that an organic solvent remained in the layer of the pressure-sensitive adhesive causes the occurrence of bubbles at the adhered interface. Furthermore, when the pressure-sensitive adhesive sheets are applied to electronic instruments such as a display, there is a possibility that the remained solvent causes the corrosion of electronic parts.
On the other hand, recently, a method of obtaining acrylic pressure-sensitive adhesive sheets by a photopolymerization has been investigated. In the method, after coating a composition composed of a (meth)acrylic acid alkyl ester as the main constituent added with a photopolymerization initiator on a base material or a releasing liner, by photopolymerizing the coated layer by the irradiation of a ultraviolet ray, etc., a layer of a pressure-sensitive adhesive is formed without using an organic solvent. The pressure-sensitive adhesive sheets produced by the method do not substantially cause problems of the occurrences of bubbles and corrosions caused by the remaining organic solvent and thus, have been expected as preferred pressure-sensitive adhesive sheets from the points of safety and an environmental hygiene.
However, in the photopolymerization type pressure-sensitive adhesive sheets, there are problems that the pressure-sensitive adhesive sheets are inferior in the adhesion to an adherend having a low polarity, such as a polycarbonate plate which is used as a protective plate for a display, etc., and bubbles generate at the adhered interface by the influence of moisture generated from the adherend, etc., to lower the appearance and the visuality. Also, when a large amount of a photopolymerization initiator is used for increasing the polymerization ratio, the initiator remains in the photopolymerization product to cause discoloring under a high-temperature atmosphere, while when the amount of the initiator is less, many unreacted monomers remain in the photopolymerization product, which cause bubbling at the adhered interface and gives a possibility of causing corrosions of electronic parts. Furthermore, depending on the surface state of a base material or a releasing liner on which the layer of the pressure-sensitive adhesive composed of the photopolymerized product is formed, the transparency is sometimes greatly reduced.